1. Field of the Invention
The present invention relates to a system for performing analysis, calculation, computation, determination, detection, estimation, measurement and/or prediction (hereinafter collectively referred to as "detection") of the state of charge (hereinafter sometimes abbreviated as "SOC") of a battery, in terms of a residual capacity, by using a saturation or full-charge voltage of the battery.
2. Description of the Related Art
The accurate estimation of the residual capacity is an important capability in an electrical vehicle which runs on battery power.
For example, in a battery residual capacity detection system disclosed in Japanese Patent Application Laid-Open Publication No.8-278351, making use of ease of calculation, a battery controller determines the residual capacity (SOC) of a battery immediately after an ignition switch is turned on, using the following expression (1), and displays this value. EQU SOC=[(Vn.sup.2 -Ve.sup.2)]/(Vs.sup.2 -Ve.sup.2)].times.100% (1),
where Vn is an estimation voltage of the battery, Ve is a discharge-end voltage of the battery, and Vs is a full-charge voltage of the battery.
Generally, in the electrical vehicle, the battery is fully charged after a travel or trip, such as when in a guarage.
During operation of an electrical vehicle, the battery has its terminal voltage and discharge current providing their time-dependent values, of which prescribed numbers are collected on associated axes of a voltage vs. current coordinate system, where they are averaged to provide a set of corresponding component data. When a prescribed number of such sets of data have been accumulated, a correlation between their component data is calculated to determine its coefficient r, and an associated regressoin line is defined as an expression for linear approximation, such that Y=aX+b, where Y and X are values on the axes of the coordinate system, and "a" and "b" are constants. As the coefficient r indicates a significant negative correlation, the regression line is determined by applying the method of least squares to calculation of the constants "a" and "b". A current value of the battery voltage Vn is estimated by subsituting a reference current Io into the expression of linear approximation.
Then, the estimation voltage Vn is substituted into the expression (1) to determine the residual capacity of the battery in terms of a current SOC during vehicle operation.
As shown in FIG. 11 for relatively high ambient temperatures, the residual capacity is calculated to be displayed as a 100% at the startup of operation (switching of ignition to on and start of driving) after a full charge. As the vehicle is driven, the residual capacity gradually decreases along a given characteristic curve.
At relatively low temperatures, however, the battery capacity becomes smaller than in a high-temperature range.
Thus, as shown in FIG. 12, in contrast to the residual capacity characteristic in the high temperature range (for example, on a summer day), it so happens in a low temperature range (for example, during the winter) that, even though the battery is fully charged, the battery voltage remains short, with a failure to display a matured residual capacity.
Under this condition, because the battery is apparently not fully charged, charging tends to be kept continued. Further, there will not be displayed an initial 100% residual capacity, and the charging might be kept long, resulting in possible deterioration of the battery.
Even if a correction is made for calculation to display an initially 100% residual capacity after a full charge in a low temperature range, a full-charge voltage Vs to be used therefor upon completion of an associated full charge is to be used as a constant in the expression (1). Thus, at low temperatures in the range, calculated values of the residual capacity describe another tendency than a gradual drop from the 100%, as illustrated by a residual capacity characteristic A in FIG. 13.
For example, with a full charge at a low temperature, the characteristic A would indicate a gradual decrease from a 80% residual capacity, when driving. Initially after the full charge, however, there would be a sudden decrease (a) from a 100% to lower than the 80% (for example, to a 70%), before entering the gradual decrease.
At low temperatures in the ragne, therefore, even if the battery has a full charge, the displayed residual capacity will be lower, so that the driver will think that the drivable distance is shorter than it actually is.
Further, even while driving, the residual capacity at part of the characteristic curve A subsequent from the sudden decrease (a) might be by calculation based on the constant voltage Vn, thus having wrong values displayed.